To meet the ever-increasing demand for high-speed and seamless data
services in the next-generation wireless communication systems, which
are expected to provide a 1000-fold increase in the network capacity,
some techniques relying on massive multiple-input multiple-output,
ultra-dense networks, and millimeter wave communications are expected
to be the key enabling technologies [1]}. However,
such solutions will generally involve increased hardware costs and
high power consumption. With the theoretical and experimental breakthroughs
in micro electromechanical systems and metamaterials (e.g., metasurface),
reconfigurable intelligent surface (RIS), as an emerging cost-effective
technology, has recently been advocated as a powerful solution to
enhance the spectrum efficiency and energy efficiency of wireless
networks [2]}, [3]}, [4]}.
